No abstract

The activity of chitobiase over an entire molt cycle was measured in Daphnia magna; it increased by 5-fold from intermolt to premolt (when animals are in apolysis). Duration of the entire molt cycle increased about 3-fold with a decrease in temperature from 25 to 6"C, but increased by <2-fold over an go-fold difference in food concentration. There was a strong (r2 = 0.95) linear relationship between chitobiase activity and duration of the molt cycle, as modulated by food concentration, in asynchronous populations of D. magna. The relationship between chitobiase activity and duration of the molt cycle, as modulated by temperature, was also highly significant (r2 = 0.77) but nonlinear. The relationship between chitobiase activity and temperature is also nonlinear (r 2 = 0.67). A significant linear relationship between chitobiase activity and duration of the molt cycle was also found for a natural population ofDaphnia rosea (r2 = 0.88). Regressions of enzyme activity with body size and glucosamine content for three species of Daphnia indicate that differences exist between species. Chitobiase activity is an index of the proportion of animals in apolysis in a population. Because this proportion changes as a function of food concentration, but apparently not as a function of temperature, chitobiase activity is a measure of the duration of the molt cycle as limited by food concentration.